Electrotinning process

ABSTRACT

In the process of reducing oxidation in stannous tin plating baths, particularly halogen tin baths, by sparging the bath with an inert gas such as nitrogen to displace dissolved and entrained oxygen, deposition of salt at the sparger outlets is minimized or avoided by the improvement of introducing water into the inert gas stream.

United States Patent Swalheim Apr. 25, 1972 [54] ELECTROTINNING PROCESS OTHER PUBLICATIONS [72] Inventor: Donald Arthur Swalhelm, Hockessin, Del. Olin Matheson Co. Catalog 25, 1965 [73] Assignee: E. l. du Pont de Nemours and Company, f 1967 Wilmington, Del. General Chemistryby Sisler et al., 1949, page 488 First-Year College Chemistry by Lewis, 1956, page 13 [22] Filed: Dec. 8, 1969 [21] App]. No.: 883,191 Primary Examiner-John l-l. Mack Assistant Examiner-R. L. Andrews 52 us. Cl. ..204/5411, 204/28, 204/246 Black [51] Int. Cl ...C23b 5/14, C23b 5/58, C22d 3/02 581 Field at Search ..204/54, 54 R ABSTRACT In the process of reducing oxidation in stannous tin plating [56] Reerences Cited baths, particularly halogen tin baths, by sparging the bath with UNITED STATES PATENTS an inert gas such' as nitrogen to displace dissolved and entrained oxygen, deposition of salt at the sparger outlets is 2,758,075 8/1956 Swalheim ..204/54R minimized or avoided by the improvement of introducing water into the inert gas stream.

7 Claims, 1 Drawing Flgure 2 "r" J 6 w l I m E I 2 Patented April 25, 1972 INVENTOR DONALD ARTHUR SWALHEIM ATTORNEY ELECTROTINNING PROCESS BACKGROUND OF THE INVENTION oxygen. In this way oxidation of the bath chemicals is minimized and substantial operating economies are achieved. While this process has great potential value, attempts to incorporate it into commercial electrotinning lines have met with only limited success. The essential difficulty of introducing the inert gas through the sparge outlets, which must be immersed in the plating bath, has been the inherent tendency of the salts of the plating bath to form solid deposits within the gas introduction apertures, resulting in plugging and stoppage of gas flow in a relatively brief time. This deposition has been observed both in metal apertures and those constructed of fluorocarbon polymer resin, which is well-known to have little adhesion to most materials. Such deposition of salt appears to be a peculiar characteristic of the halogen tin plating bath.

SUMMARY OF THE INVENTION According to the present invention there is provided a process for the electrodeposition of tin from an aqueous stannous electroplating bath in which process an inert gas is employed to purge and displace dissolved and entrained oxygen from the bath, wherein the improvement comprises adding water to the inert gas.

BRIEF DESCRIPTION OF DRAWING In the drawing there is shown a plating installation for depositing tin on strip steel. This equipment is of the type covered by US. Pat. No. 2,490,055 issued to C. M. Hoff on Dec. 6, I949.

DETAILED DESCRIPTION OF THE INVENTION The improvement of this invention can be advantageously applied to the process disclosed in U.S. Pat. No. 2,758,075. As therein disclosed, any gas which is completely inert in the plating system can be used. There can be used, for example, carbon dioxide, carbon monoxide, argon, helium, hydrogen, and hydrocarbons such as methane, ethane, propane and butane. It will be understood that carbon monoxide must be very carefully handled because of its toxic effects. Hydrogen and hydrocarbons also must be handled with great care because of their flammability. By far the preferred gas for use according to the invention is nitrogen. In the following description nitrogen is used as illustrative of the useful inert gases.

In use, stannous baths ordinarily contain dissolved and dispersed air. The amounts of such dispersion air vary depending partly upon the extent of agitation of the bath. Normal operating practice effects a rather large amount of agitation and aeration so that the solutions usually contain large amounts of air.

Any gases present in the baths will be dissolved to the extent of their solubility and additional amounts may be present as finely dispersed bubbles of undissolved gas. The usual stannous plating baths will hold large amounts of dispersed gases. Halogen-tin baths, for example, if agitated in the presence of air become filled with finely dispersed air bubbles which are only slowly released. A brief description of a commercial electrotinning operation will be useful in appreciating the problem.

Most commercial units are operated at strip speeds in the range of 1,500 to 2,000 ft./min. A sketch of a typical plating cell is shown in the drawing. The direction of travel of the strip 8 is from left to right as it is gripped by entry rolls 6 and 7 and drawn by rolls 1 and 2. The strip pumps" the electrolyte toward the exit end of the cell (towards rolls 1 and 2) with the result that it piles up to a height of several inches near rolls 1 and 2 when the strip travels at such speeds. The pile-up normally extends back several feet from rolls 1 and 2 and tends to flow back toward the entry end of the cell. The net result is that the electrolyte flows in a rolling" type of pattern. The shearing action of the strip has an effect similar to aspiration. At high strip speeds air becomes entrained through aspiration near the surface of the electrolyte. The shearing" or pumping action of the strip at these high speeds breaks up larger bubbles of air into extremely small dispersed air bubbles which remain suspended and are only slowly released from theelectrolyte. Through this action the concentration of oxygen in the electrolyte is brought close to saturation. In order to effectively utilize an inert gas to reduce the dissolved and dispersed oxygen content, the displacing inert gas should also be dispersed as finely divided bubbles. The finely dispersed bubbles tend to remain suspended in the electrolyte, whereas large bubbles rapidly escape to the atmosphere. The high surface area resulting from small bubbles probably aids in dissolving appreciable gas in the bath and thereby reducing the concentration of dissolved oxygen.

The inert gas can be introduced into the system by any means suitable to accomplish the objectives of this invention. The simplest apparatus would consist of one or more openended tubes or small I.D. pipes. A distribution pipe with small holes is frequently used to introduce a gas into a liquid. Another common method is to introduce the gas through a porous material, which offers the advantage of introducing the gas in the form of finely dispersed bubbles. Passing the gas, for example, through porous carbon produces very small gas bubbles. As indicated earlier, the inert gas must be properly dispersed into the electrolyte in order to be most effective in reducing the oxygen concentration in the bath. Referring to the drawing, electrolyte is continuously pumped to the plating cells through pipes 3 and 4 shown at the entry end, and is withdrawn and returned via pipe 5. Good dispersion of the gas can be accomplished by introducing the gas into pipe 4 because of the shearing action of the strip. The inert gas can also be introduced into the downcomer or returnJine 5 by means of an inserted pipe 9 fitted with a porous or perforated end piece to produce finely dispersed gas bubbles.

The improvement of this invention can be utilized in any stannous electrodepositing bath which contains part or all, and preferably at least a major amount, of its tin as a stannous compound. Prior art baths can be used in which tin is present as stannous chloride, stannous sulfate, stannous fluoride, and phenol sulfonate or other stannous salts of sulfonic acid. Baths of these types frequently include acids such as hydrochloric, hydrofluoric, sulfuric acid, or aromatic sulfonic acids. Typical stannous baths are described in U.S. Pat. No. 2,457,152 issued to Raymond A. Hoffman on Dec. 28, I948, and in patents cited therein. Also suitable are such baths as the sulfonate baths described in U.S. Pat. No. 2,399,194 issued to J. W. Andrews on Apr. 30, 1946; U.S. Pat. No. 2,450,794 issued to Elmer F. Harris on Oct. 5, i948; and U.S. Pat. No. 2,450,795 issued to Elmer F. Harris on Oct. 5, 1948.

The preferred stannous baths for use according to the present invention are the so-called halogen-tin" plating baths which are chiefly composed of an alkali fluoride-stannous chloride solution. These baths are described and claimed in U.S. Pat. No. 2,407,579 issued to Ernest W. Schweikher on Sept. 10, I946.

The invention comprises introducing water in the form of vapor or liquid with an inert gas to prevent salts forming and building up at orifices or other types of outlets used to introduce inert gases into stannous tin plating baths, particularly those containing chloride or fluoride ions.

This invention can be applied over a broad range of gas temperatures, from near 0 C. to about C., and preferably from about 60 C. to 125 C. In practical terms the temperature of the inert gas should be high enough to dissolve and/or carry entrained an effective quantity of moisture. Of course, the gas temperature can also be chosen to suit the operating conditions of the plating bath. Ordinarily the baths will be used at comparatively high temperatures, that is above $-55 C. in practical operation the problem is usually one of cooling the bath enough to keep the temperature below 75 C. This problem is particularly great when a bath, such as the halogen-tin bath, is used at very high current densities for high speed operation.

The flow rate of the inert gas should be chosen with due consideration to the amount of oxygen contained in the plating bath which is sought to be removed by the inert gas, the quantity of moisture to be introduced, the speed of the sheet to be plated, the geometry and location of the gas introducing apertures, and other relevant mechanical and operational factors.

The quantity of moisture to be introduced likewise will depend upon such considerations as the tendency of the particular salts to deposit, the particular apertures used, the composition of the bath, the flow rate of the inert vant factors.

Moisture can be introduced into the inert gas by any suitable means. Such would include bubbling the inert gas through a water reservoir, spraying water into the inert gas stream, passing the inert gas through one of the several available types of evaporative humidifier, injecting saturated or superheated steam into the inert gas stream, or by any other technique appropriate under the circumstances for introducing water vapor, condensate or liquid into a fluid stream. I It is understood that such factors as quantity and temperature of moisture, flow rate and temperature of inert gas, design and construction of moisture adding devices and inert gas introducing devices are operational considerations which do not limit the scope and spirit of this invention and are best deter mined by experimentation reflecting the actual requirements of the users process.

The invention will be better understood by reference to the following examples. in all cases the electrolyte used had the following composition: SnCl 45 g./l NaF 45 g./l., NaHF gas, and other rele- 38 400 59 400 67 390 75 275 78 H0 83 Zero EXAMPLE 2 H16 in. ID. Tube-Initial Flow of N =5 ml./min.

EXAMPLE 3 1/16 l.D. Tube-Initial Flow of N,275 ml./min.

It is evident from the results shown in control Examples 1, 2,

and 3 that there is restriction to flow within a relatively short time. Examination of the tubes after plugging showed salt 2 ml./ 1. of a mixture of polyethylene oxide and ethylene glycol monoethyl ether. This composition is typical of those used in commercial halogen tin plating operations. The electrolyte was placed in a 1,500 ml. stainless steel beaker; volume of the electrolyte was maintained constant at 1,000 ml. Temperature of the electrolyte was controlled 60 C. 1 C. by means of a constant temperature water bath. In these experiments the inert gas used was nitrogen. This was introduced into the electrolyte by means of a polypropylene tube having an inside diameter of one-sixteenth, one-eighth or three-sixteenths of an inch. Restriction of flow of nitrogen through the tube was followed by measuring the rate of flow of nitrogen; pressure of nitrogen was maintained constant at 6 psi. The data in the examples are nitrogen flow rates as a function of time. Decline in flow rate indicates restrictive build-up of salt deposit within the polypropylene tube.

Examples 1, 2, and 3 are controls in which no moisture was introduced into the nitrogen. Their purpose is to demonstrate the inherent tendency of the inert gas introducing lines to plug when the improvement of this invention is not used.

EXAMPLE l is in. ID. Tube-Initial Flow of N =O ml./min.

build-up within the tubes extending from about V4 inch from the end to about 1 inch from the end of the tube. In other words, salt formed over a "94 inch distance within the tube. These control examples further indicate that the geometry of the gas injecting device has an effect on salt build-up which should be taken into account in designing apparatus, as noted above.

ln Examples 4 to 9 the nitrogen was bubbled through water prior to introducing it into the tin plating bath. The water was contained in a 250 m1. suction flask which also contained 5s inch diameter plastic balls. The presence of the plastic balls provided good contact of the nitrogen bubbles with the water, thereby insuring maximum pick-up of moisture. The temperature of the water was controlled at the levels shown.

EXAMPLE 4 Passing N Through H O at 60 C. 1/16 in. ID. Tube In Plating After terminating the experiment, examination of the tube showed a tiny hole in the center which was not plugged with salt. The salt extended from A inch from the end to a distance of 1 inch from the end. It can be concluded that the moisture in the nitrogen was effective in preventing the tube from plugging completely.

Examination of the tube at the termination of the test showed that the tube was essentially free of salts. The very small amount of salt found on one wall would not restrict flow of nitrogen. This example was repeated with water at 7 l .l C. No trace of salt was found in the tube after operating for a period of 200 minutes.

Examples 6 to 9 show water temperatures below 60 C.

EXAMPLE 6 Passing N Through H O at 233 C. H16 in. ID. Tube in Plating Solution Time, min. Flow Rate, ml./min.

Initial 550 I3 550 42 500 65 490 107 450 136 365 138 275 178 275 193 Zero EXAMPLE 7 Passing N Through H O at 23.3 C. H16 in. ID. Tube in A comparison of the results shown in Examples 6 and 7 with the results shown in control Examples 2 and 3 indicates that passing N through water at 23.3 C. has a beneficial effect in slowing down the rate at which salts form in the tube.

EXAMPLE 8 Passing N Through H O At 37.8 C. III 6 in. [.D. Tube ln No further improvement in preventing plugging was shown by passing N through water at 37.8 C. as indicated by the results given in Example 8.

EXAMPLE 9 Passing N Through H O At 48.9 C. Ill 6 in. ID. Tube ln Plating Solution Time, min. Flow Rate, ml./min.

Initial 275 I3 275 29 275 89 275 106 225 136 205 I78 190 213 273 335 391 I75 450 140 571 165 The tube was not completely plugged after 571 minutes. Examination of the tube showed a salt plug similar to that described in Example 4.

In consideration of all the above examples, it is apparent that the formation of salt plugs in tubes used to introduce nitrogen into stannous tin baths containing fluoride and chloride, as defined by compositions claimed in U.S. Pat. No. 2,407,579, can be prevented or minimized by humidifying the nitrogen. Best results were obtained by passing the nitrogen through water maintained in excess of 60 C. Under these conditions, small amounts of condensate are introduced simultaneously with the nitrogen. The warm condensate passing through the tube apparently inhibits the salts from depositing on the inner walls of the tube.

In Example 10, a syringe was used to inject liquid water directly into the polypropylene tube. A tee was inserted in the nitrogen flow line directly above the polypropylene tube with the syringe in a .vertical position directly above the polypropylene tube. This arrangement permitted injecting water down into the tube while room temperature nitrogen was bubbling into the plating solution. The water temperature was approximately 23 C.

EXAMPLE l0 Injecting Water With The Nitrogen I/ I6 in. ID. Tube In Plating Solution Time, Flow Rate Water Injected min. ml./min. ml. Remarks Initial 275 I 275 0.2 20 275 0.2 30 275 0.2 40 275 0.2 50 275 0.2 60 275 0.2 70 275 0.2 80 275 0.2 90 275 0.2 I00 275 0.2 I I0 275 0.2 I20 275 0.2 I30 275 0.2 I40 275 0.2 I50 275 0.2 I60 275 0.2 I70 275 0.2 Appreciable salt in tube 180 275 0.2 220 190 225 190-275 0.2 Water increased flow rate 235 275 0.2 245. 275 0.2 255 275 0.2 Very little salt 265 275 0.2 Very little salt The results of Example 10 demonstrate that periodic additions of water are effective in preventing the tube from plugging. Similar results were obtained by substituting a Ainch l.D. tube for the H16 inch I.D. tube of Example 10.

In Examples 11 and 12, steam was injected with the nitrogen. A hot plate was used to heat the 250 ml. suction flask to temperatures higher than 100 C. When water was introduced into the flask by means of the syringe, the water vaporized into steam with no visible condensation on the walls of the flask.

EXAMPLE 1 l Injecting Steam With The Nitrogen l/l6 in. ID. Tube- Temperature of Steam l04.4 C.-I2 l .l C.

Periodic additions of steam were also effective in preventing plugging of the tube as shown by the results of Example 11.

EXAMPLE 12 Injecting Steam With The Nitrogen Larger 3/16 in. ID. Tube- Steam 104.4 C.-l2 I 1 C.

Time, Flow Rate Water Injected min. ml./min. ml. Remarks Initial 550 I0 550 0.2 20 550 0.2 30 550 0.2 40 550 0.2 50 550 0.2 60 550' 0.2 Some salt in tube 70 550 0.2 550 0.2 550 v 0.2 I00 550 0.2 I I0 -550 0.2 I20, 550' 0.2 I30 550 0.2 Appreciable salt in tube 550 0.2 I50 550 0.2 Heavy salt, tiny hol'e in center I53 550 0.2 Most of salt removed I55 550 0.2 Very little salt in tube I65 550 Appreciable salt in tube I66 550 0.6 Three successive additions of 0.2 ml. water required to remove all the salt Steam was also effective when introduced with nitrogen into a larger I.D. tube as shown by results in Example 12.

What is claimed is:

1. In a process for the electrodeposition of tin from an aqueous stannous electroplating bath, in which process an inert gas is introduced into the bath liquid by at least one orifice to purge and displace dissolved and entrained oxygen from the bath, wherein the improvement comprises adding water to the inert gas in an amount sufficient to reduce salt formation at an orifice used to introduce the inert gas into the bath liquid.

2. The process of claim 1 wherein the stannous electroplating bath is a halogen tin bath.

3. The process of claim 1 wherein the inert gas is nitrogen.

4. The process of claim 1 wherein the water is present in the inert gas as dissolved vapor, condensate, drops, entrained liquid, or any combination thereof.

5. The process of claim 1 wherein the temperature of the water-containing inert gas is within the range of about 0-I 25 C.

6. The process of claim 1 wherein the water-containing inert gas is added so as to create a multiplicity of finely dispersed bubbles in the bath liquid.

7. In a process for the electrodeposition of tin from an aqueous halogen tin electroplating bath, in which process nitrogen is introduced as finely dispersed bubbles into the bath liquid by at least one orifice to purge and displace dissolved and entrained oxygen from the bath, the improvement which comprises, adding water to nitrogen in an amount sufficient to reduce salt formation at an orifice used to introduce the inert gas into the bath liquid and adding the water-containing nitrogen to the bath at a temperature within the range of about 60-l25 C. 

2. The process of claim 1 wherein the stannous electroplating bath is a halogen tin bath.
 3. The process of claim 1 wherein the inert gas is nitrogen.
 4. The process of claim 1 wherein the water is present in the inert gas as dissolved vapor, condensate, drops, entrained liquid, or any combination thereof.
 5. The process of claim 1 wherein the temperature of the water-containing inert gas is within the range of about 0*-125* C.
 6. The process of claim 1 wherein the water-containing inert gas is added so as to create a multiplicity of finely dispersed bubbles in the bath liquid.
 7. In a process for the electrodeposition of tin from an aqueous halogen tin electroplating bath, in which process nitrogen is introduced as finely dispersed bubbles into the bath liquid by at least one orifice to purge and displace dissolved and entrained oxygen from the bath, the improvement which comprises, adding water to nitrogen in an amount sufficient to reduce salt formation at an orifice used to introduce the inert gas into the bath liquid and adding the water-containing nitrogen to the bath at a temperature within the range of about 60*-125* C. 